https://leetcode.cn/problems/binary-tree-pruning/description/
class Solution {
public:
    TreeNode* pruneTree(TreeNode* root) {
        if(root==nullptr)
            return nullptr;
        if(root->left)
            root->left=pruneTree(root->left);
        if(root->right)
            root->right=pruneTree(root->right);
        if(root->left==nullptr&&root->right==nullptr&&root->val==0)
        {
            delete root;
            return nullptr;
        }    
        return root;

    }
};